I wondered if it was part of the plan.
I should imaging that Stonehenge was a bit of a project in its day and, as the end user, I should imagine the denizens of what became Wiltshire, wanted to make sure they were getting a functional computer/calendar/ precursor to the electronic thumb (I guess they knew where their towel was).
Perhaps the persuaded Bran the Builder to demonstrate it before they paid for it (inclusive of carriage of course)?